21044699 Design of Steel Footbridges 2005

The design of steel footbridges

Corus Construction & Industrial

Steel bridgesthe gap

Below:River Aire footbridge, Leeds, 1993Right:Lowry Footbridge, Manchester

The design of steel footbridges 3

Contents1. Introduction

2. Features and forms of construction

for footbridges

3. Conceptual design and detailing

3.1 General arrangement

3.2 Selection of type of construction

3.3 Trusses and vierendeel girder bridges

3.4 Steel beam bridges

3.5 Composite beam bridges

3.6 Cable stayed bridges

3.7 Access ramps and stairs

3.8 Bearings and expansion joints

4. Design codes, standards and guidance

4.1 British Standards

4.2 Departmental standards

4.3 Railway standards

4.4 Design of hollow section joints

4.5 Design of cable stayed and suspension bridges

4.6 Design of steel and composite bridge beams

4.7 Dynamic response

4.8 Protective treatment

4.9 Steel materials

5. Flow charts

6. References

This guide has been prepared for Corus by:

D C Iles MSc ACGI DIC CEng MICE Manager – Bridges,

The Steel Construction Institute.

The author gratefully acknowledges the contributions

made by Mr W Ramsay, Corus and Mr A C G Hayward,

Cass Hayward and Partners, during the original

preparation of the publication.

4 The design of steel footbridges

Introduction

Footbridges are needed where a separate pathway has

to be provided for people to cross traffic flows or some

physical obstacle, such as a river. The loads they carry

are, in relation to highway or railway bridges, quite

modest, and in most circumstances a fairly light

structure is required. They are, however, frequently

required to give a long clear span, and stiffness then

becomes an important consideration. The bridges are

often very clearly on view to the public and therefore the

appearance merits careful attention.

Steel offers economic and attractive forms of

construction which suit all the requirements demanded

of a footbridge.

A fully detailed design can be prepared with other

contract documents for pricing by tenderers. However, it

is common practice, particularly for smaller bridges, for

the detailed design of a footbridge to be included as

part of a design and construct package. Many

fabricators are able to provide such a package, using

methods and details of construction developed to suit

their particular fabrication facilities and expertise.

However, the engineer supervising the work still needs

to be acquainted with the different forms of construction

which might be used and to be aware of their

advantages and limitations.

Longer span bridges and those which form part of a

larger scheme are likely to be designed in detail by a

consultant or local authority. Within such an

organisation the engineer carrying out the design needs

to be familiar with the particular requirements for

footbridges, their features and construction details.

For the engineer in either of these situations, this

publication presents guidance on the conceptual design

of steel and composite footbridges, to aid the selection

of an outline design.

Typical key features are illustrated in section 3,

references to codes and sources of further guidance are

given in section 4. Simple flow charts showing the

design steps are presented in section 5.

1. Introduction


Features and forms of construction for footbridges

2. Features and forms of construction for footbridgesBasic requirements

Footbridges, like any other bridge, must be long enough to

clear the obstacle which is to be crossed and high enough

not to interfere with whatever passes beneath the bridge.

However, the access route onto the footbridge is often

much different from what is familiar to the designer of a

highway bridge: there is no necessity for a gentle horizontal

alignment (indeed the preferred route may be sharply at

right angles to the span). Structural continuity is therefore

less common. The principle span is often a simply

supported one.

Provision of suitable access for wheelchairs and cyclists is

often specified for footbridges. Access ramps must be

provided and restricted to a maximum gradient. The

consequent length of ramps where access is from the level

of the road or rail track over which the bridge spans is

generally much longer than the bridge itself. The form of

construction suitable for the ramps may have a dominant

influence on the final form of the bridge.

The width of a footbridge is usually quite modest, just

sufficient to permit free passage in both directions for

pedestrians. Occasionally the bridge will have segregated

provision for pedestrians and cyclists, in which case it will

need to be wider.

Parapets are provided for the safety of both the pedestrians

and traffic flow. Footbridges over railway lines are required

to have higher parapets and be provided with solid panels

directly over the rail tracks.

Truss and vierendeel girder beams

Trusses offer a light and economical form of construction,

particularly when the span is large. The members of the

truss can be quite slender and this naturally leads to the

use of structural hollow sections. Hollow sections have

been used for footbridges for over 30 years and some

fabricators have specialised in this form of construction,

developing techniques and details which utilise them to the

best advantage.

Vierendeel girders using hollow section members offer an

alternative but complementary structural form of similar

proportion by substituting a rectangular form for the

triangular arrangement used in trusses.

Trusses and vierendeel girders are arranged with either

half-through or through construction. Half-through

construction is used for smaller spans, where the depth

needed is relatively shallow. For larger spans, or where the

truss is clad to provide a complete enclosure for the

pedestrians, through trusses are used; the top chords are

then braced together above head level.

Steel beam bridges

The simplest method of employing structural steel as the

prime structural element of a footbridge is to use a pair of

girders (fabricated or rolled sections), braced together for

stability and acting as beams in bending, with a non-

participating walkway surface on top. A typical small

bridge deck might for example be formed by timbers

placed transversely across the top of the beams. Precast

slabs might also be used, without being shear connected

to the steel and therefore not participating in global

structural action.

Left:Bell’s Bridge, GlasgowRight:Whatman’s Field Bridge, Maidstone


Alternatively the floor might be formed by steel plate,

suitably stiffened to carry the pedestrian loads, in which

case the plate could also be made to act structurally as the

top flange of the steel beams.

Steel box girder bridges

Another alternative is to use a small steel box girder. The

top flange acts as the floor of the bridge, and there are

usually short cantilevers either side of the box. This form

has the benefits of good torsional stiffness which can

simplify support arrangements and clean surfaces which

minimise maintenance.

Composite beam bridges

Composite beams, steel girders with a concrete slab

acting as both a walkway floor and participating as a

top flange, are a practical solution for medium span

footbridges. They are a lighter version of the form of

composite construction frequently employed in

highway bridges. Slabs may be cast insitu, though the

lesser requirements for the shear connection and the

lighter design loads on the slab allow greater

opportunity to employ pre-cast slabs. The slab can also

be cast on the beams in the works or other convenient

site, since the weight and dimensions are often

sufficiently modest to permit transport and erection of

the complete superstructure.

Although composite construction is usually associated

with I section girders, a concrete slab can also be used

with a steel box girder.

Cable stayed bridges

In seeking to provide a bridge of light appearance, the

use of cable stays is found to be very successful. It

often affords scope to create a visually striking structure

which provides a landmark or a focus for the area in

which it is located. Almost any form of construction can

be used with stays, though when a cable stayed form is

chosen, the structural requirements are often found to

be of secondary consideration to the achievement of a

pleasing appearance.

Enclosed bridges

Enclosure of the sides of a footbridge is often called for

to discourage the throwing of objects from the bridge.

This is a particular requirement for bridges over railway

lines. Full enclosure, to the sides and the roof of the

walkway, is called for in situations where the users are

to be protected from the environment and where greater

protection is required over railway lines. Such enclosure

justifies the use of through truss or vierendeel

construction. The form of construction will probably be

dictated by consideration of appearance of the bridge

and its relationship to adjacent structures. Whilst the

general principles discussed in this guide are

applicable, fully enclosed bridges are not specifically

dealt with in detail in this guide.

Decorative features

In addition to the basic impression made by the form of

construction, the appearance can be greatly influenced

by non-structural decorative features, such as parapets

and handrails. Where particular effects are sought, the

availability of different patterns for posts, rails, etc,

should be investigated. Non-structural embellishments

of supports can also contribute – for example a cable

stayed pylon can be extended to a spike or other feature

above the level of the topmost stay connection.

Landmark structures

It is an increasingly common requirement for footbridges

in prominent or key locations to be ‘landmark

structures’. Particular attention is given to the

appearance of the structure and this may result in

somewhat unusual forms of construction. Such

structures can be allowed to be marginally less efficient

(in terms of complexity of fabrication), but if the design

is well executed the penalties should be small.

There is more scope for innovative design when the

structure is not over a road or railway, because the

requirements for parapet details need not be so

stringent. Parapets are often the most noticeable feature

of a footbridge, and the freedom to use more attractive

forms and more open post and rail arrangements can

lead to a very pleasing appearance.

The use of curved arch-type members is currently quite

popular, as is the use of cable stays. Some recent

examples are illustrated on this page.

Since these landmark structures are generally innovative,

it is inappropriate to try to include design guidance here,

but the general requirements and design principles given

in the following sections are largely still applicable.


Features and forms of construction for footbridges

Left:Swansea Sail BridgeBelow:Halfpenny Bridge, SheffieldRight:Millennium Bridge, Gateshead

3. Conceptual design and detailing

3.1 General arrangementAs a first step, the basic requirements for access and

safety should be determined. The width and form of

access needed depends on the expected pedestrian

traffic flow, though minimum dimensions are adequate in

most cases.

For a simple footway, a minimum clear width of 2.0m is

required by the highways authorities. Railway station

footbridges can be less wide. To the sides of this

footway, parapets are required, which should be 1.15m

high over roads or 1.5m high over railways, the height

measured from the footway surface in both cases. In

areas prone to vandalism, a height of 1.8m may be

required over railways. The resulting minimum cross

section to be provided is shown in Figure 1. An

increased parapet height of 1.3m may be needed in

areas of high prevailing wind and for bridges where the

headroom under the bridge is more than 10m.

Where pedestrians and cyclists share the pathway, the

minimum width of 2.0m may be used for low traffic

flows but a wider segregated pathway (1.5m + 1.5m

minimum) may be required for higher traffic flows.

Segregation can be achieved by a white line, colour

contrast or difference in surface texture. At the same

time the minimum parapet height is increased to 1.4m.

The cross section for a combined pathway is also

shown in Figure 1.

Dimensional requirements for footbridges are given in

Departmental Standard BD 29/03. That document refers

to BS 7818 for minimum dimensions of parapets.

The drainage requirements also affect the cross section,

since kerbs will be needed to prevent run-off where the

bridge is above a carriageway, a footpath or rail tracks.

Typically an upstand of 50mm should be provided. This

upstand can be provided by an edge beam, by the lower

chord of a truss or by a flat welded to the floor plate.

Figure 1: Basic sectional dimensions for bridges over highways


Conceptual design and detailing

Footway Cycleway

1.5m 1.5m

1.4m

Marked segregation

Minimum footway

2.0m

1.15m

Footway + cycleway

2.0m

1.4m

4.5m

5.7m

Span

Since there is usually no need to align the approaches

to a footbridge, the span should normally be arranged

square to the obstacle it has to cross.

The minimum span required is that simply needed to

clear the width of obstacle, carriageway or railway.

However, the span may be increased in order that the

supports are positioned far enough from a carriageway

or rail track to avoid the risk of impact from an errant

vehicle or derailed train. The supports of light structures

such as footbridges are particularly prone to the effects

of impact.

For footbridges over highways, the span is determined

by the dimensions of the carriageways, as given in the

Departmental Standard TD 27/96.

To avoid the imposition of impact loads the supports

need to be set back 4.5m from the edge of the

carriageway (see Figure 2). Where this can be arranged,

perhaps additionally spanning a footway beside the

road, the consequent savings in the cost of the

substructure should be considered. Supports between

carriageways should also be avoided if possible.

The space needed for approach ramps and stairs will be

significant in arranging the layout of a footbridge. This

may influence the positioning of the bridge and its

supports, and thus its span.

Footbridges over railways are mostly required to cross

two or four tracks, with resulting span of between 10

and 25m. Where intermediate supports are placed

closer than 4.5m to the nearest rail, Network Rail require

the superstructure to be capable of supporting itself if

one support were to be demolished in an accident.

Clearance

Over a highway, the clearance under new footbridges is

required to be at least 5.7m (TD 27/96). With this

clearance the superstructure need not be designed for

impact loads (see Figure 2). If any relaxation on

clearance were permitted in special cases it is likely that

impact loads would have to be considered. This would

be very onerous on the structural design. Clearance over

railways is specified by Network Rail with a minimum of

4.640m from rail level. The minimum clearance over

electrified lines and over lines that might be electrified in

the future is 4.780m. Greater clearances are required

near level crossings and where there is ‘free running’

(where the wires are not attached to the bridge).

Clearly, where access to the bridge has to come from

carriageway or track level, the rise needed for the stairs

or ramps is the sum of the clearance plus the

superstructure construction depth (walkway surface to

structure soffit). This means that ramps will be long

(about 120m at each end of the bridge over a road, for a

1 in 20 grade). It also means that the depth of

construction (for example the depth of a plate girder)

can add significantly to the length of ramp, and thus to

the cost of the whole structure. For this reason, half-

through construction, with a very shallow construction

depth, is usually preferred.

Sufficient vertical camber is needed to ensure drainage

of the footbridge to the ends, where the run-off can be

carried to drains or a soakaway.

Figure 2: Governing dimensions in elevation





Stairs and ramps, ChristchurchSpiral ramp, Myton Footbridge, Hull

Stairs and ramps

Where access is required from a lower level, stairs and

ramps must be provided. Stairs are only suitable for able

pedestrians and it is general policy to provide ramps

where possible. Such ramps should ideally be no steeper

than 1 in 20, though gradients of up to 1 in 12 may be

used for straight ramps where space is limited.

A ramp can be either a series of straight sections or a

spiral, depending on circumstances and space available

(see Figure 3). The space occupied by a ramp is quite

significant and may well influence the position of the

bridge.

A single straight ramp can be used where space and the

desired access route permit. If the gradient is steeper

than 1 in 20, the ramp should have intermediate landings

(i.e. it should be a series of ramps with horizontal

sections between). Ramps are often arranged in scissor

fashion (i.e. with a 180º change of direction at an

intermediate landing).

Spiral ramps must have a minimum inside radius of

5.5m (gradient measured 900mm from the inside edge).

The same limits on gradient apply (i.e. a maximum of 1

in 20 is desirable, up to 1 in 12 may be acceptable in

some cases). Spiral ramps are unsuitable for a full 6m

rise to a footbridge over a highway unless a large radius

can be accommodated.

Stepped ramps are sometimes used which, with a

125mm step and a 1 in 12 slope between, can effectively

achieve a 1 in 6 gradient. For spiral ramps this gives a

rise of 6m in under 360º turn.

Stairs are usually arranged in two or three flights with

intermediate landings, depending on particular

arrangements, to comply with normal safety

requirements. They usually have semi-open risers, for

lighter appearance. Handrails are provided on the inside

faces of the parapets on stairs and ramps. Minimum

widths must be maintained between these handrails.

Services

Occasionally the bridge may have to carry a service –

water pipes or electric cables, for example. It should

normally be arranged that such pipes are supported out

of sight, on brackets or cross-members between main

beams for example. If a service is positioned inside a box

girder, it is better to put it in a duct, so that any

maintenance to the service does not require entry into the

box girder. Gas or water pipes should not be sited inside

a box girder, for safety reasons, unless placed in a steel

sleeve which runs the length of the bridge.


River Exe Suspension Bridge

3.2 Selection of type of constructionAs mentioned previously, the depth of construction is

very important to the overall extent of the footbridge

where access is from the level of the road or railway

being crossed. In those circumstances it is usually

preferable to use a half-through form of construction.

This usually leads to a selection of a truss or vierendeel

girder bridge, though half-through plate girder forms such

as that developed by Network Rail may also be used.

However, not all bridges are subject to such constraints.

Some simply cross, for example, a small river, or span

across a deep cutting. In such cases the depth of

construction is not so important and steel girders or steel

composite construction may be employed. When the span

is long, the dynamic response of the bridge becomes a

significant consideration, particularly for the lighter all-

steel bridge. The greater stiffness afforded by truss

construction may well be advantageous. Alternatively,

cable stayed construction can be employed.

Cable stayed forms of construction can rarely be

justified visually below about 40m. For spans up to

100m a single pylon on one side of the main span is

often appropriate, both visually and structurally. Beyond

about 100m twin pylons should be considered.

Suspension bridges are very rarely considered these

days, but may still be chosen for appearance reasons

when the span exceeds about 70m.

A summary of approximate span ranges suitable for the

various types is given in Table 1.

Table 1

Span ranges for different types of construction

Construction type Span range (m)

Truss 15 to 60

Vierendeel girder 15 to 45

Twin steel girders 10 to 25

Steel girders + steel floor plate 10 to 30

Steel box girder 20 to 60

Composite beams 10 to 50

Arches 25 upwards

Cable stayed bridge 40 upwards

Suspension bridge 70 upwards

13 risers max

1:20

1:20

≥ 2m

Figure 3: Arrangement of typical stairs and ramp



3.3 Trusses and vierendeel girderbridgesAlthough trusses and vierendeel girders have a different

structural action, there are many similar features when

they are constructed of structural hollow section

members, as used in footbridges. This section deals with

both types of construction.

Through and half-through construction

Trusses and vierendeel girders for footbridges are

normally arranged with the deck at the level of the

bottom chord, in either through or half-through

construction. Half-through construction is used for

smaller spans, where the depth needed is less than the

clearance height for people to walk through. For large

spans, or where the bridge is clad to provide a

complete enclosure for the pedestrians, through

construction is used.

The top chords can then be braced together above

head level.

Stability of the top compression chord in half-through

construction is provided by the U-frame action of the

side members and the cross-members of the deck. In

through construction, lateral bracing between the two top

chords offers a more direct means of stabilising them.

Below and right:Through truss footbridge



Configuration

The type of truss usually employed is either a Warren

truss or a modified Warren truss. Occasionally a Pratt

truss may be used. The different types are illustrated in

Figure 4.

Warren trusses are the simplest form of truss, with all

loads being carried principally as axial loads in the

members and with the minimum of members meeting at

joints. However, the loads which are carried to the

bottom chords from the walkway floor can lead to

significant bending in these members when the panels

are large. A modified warren truss reduces the span of

these chord members, though the additional vertical

members add complexity to the fabrication. Pratt trusses

are used where it is preferred that some members are

vertical, for example to facilitate the fixing of cladding or

decorative panels.

Vierendeel girders have no diagonal members and rely

on a combination of axial loading and bending to carry

loads. The stiffness of the girder depends crucially on

the bending stiffness of vertical and horizontal members

and on the stiffness of the joints between the two. As a

consequence they are much heavier, for a given span,

than a Warren truss. However the appearance, which

only shows vertical and horizontal lines, in harmony with

the normal form of parapet (horizontal rails, vertical

posts and infill), is often considered more pleasing.

For the largest spans, the vierendeel girder will probably

be too flexible, though they have been used successfully

up to 45m span.

Below:Half-through truss footbridge

Below:Rutherglen station footbridge

Figure 4: Types of truss and vierendeel girder

Pratt truss

Modified Warren truss

Warren truss

Vierendeel girder


Above:Large-span truss footbridgeLeft:Vierendeel footbridgeRight:Lower chord connection detailFar right:Large-span vierendeel footbridge, A27 Broadmarsh

Proportions and appearance

The familiar image of a truss is probably of a heavy-

looking structure, relatively deep in proportion to span.

Such trusses were often used for railway bridges.

However, a truss footbridge can generally be of light

appearance and of shallow depth/span proportion.

With half-through construction, the minimum overall

depth is determined by the parapet height; for a

crossing over a highway the minimum is about 1.25m.

For spans over about 30 metres the depth will need to

be slightly greater, though span/depth ratios in excess

of 30 can give a pleasing appearance.

For spans over 50m full through construction will

probably be necessary. Then the depth is determined by

internal clearance, which is usually specified as 2.3m

minimum. To reduce the tunnel effect and to keep the

top bracing away from casual abuse a depth of about

3m is needed. Such spans will have a deeper

span/depth ratio, though the slender members will still

give an impression of lightness.

The arrangement of the bracing and the line of the

parapets are the dominant features which are seen

by road users. They therefore require careful attention

and treatment.

Where the depth of the vierendeel girder is determined

by parapet height, the top chord can often be used as

the parapet rail, with suitable infill bars fixed between

the vertical members. For longer span vierendeel

girders, where the depth is more than the parapet

height, parapet panels complete with top rail can be

fixed inside the rectangular panels of the girder. Where a

truss is used, the parapet is usually fixed to the inner

face of the diagonal members. The parapets are less

conspicuous to road users than the truss members,

though they are still evident in silhouette.

Construction depth, from footway surface to underside

of the truss or girder, is normally quite shallow, not more

than the depth of the chord members. This contributes

greatly to the light appearance.

The top and bottom chords of a truss are usually made

parallel, but for larger spans a less dominating

appearance can be achieved by a hog-back

configuration, with a gentle curve to the top chord

reducing the depth at the ends of the span.



Members and connections – trusses

Both circular and rectangular structural hollow sections

are commonly used in trusses. The bottom chord is

generally rectangular, to facilitate connection with deck

and cross-members. Rolled sections or flats are

sometimes used as cross-members or as stiffeners to

steel floor plates. Chords and diagonals are usually

arranged with centrelines intersecting where possible.

Standard welding details have been developed for

hollow section connections.

For half-through trusses the connection with

cross-members at the lower chord requires particular

attention, since its stiffness and strength are

fundamental to U-frame action.

Where the bottom chords are of rectangular section,

some designers specify plates slotted diagonally across

the section at the position of the cross-members (Figure

5) to prevent the chord lozenging or distorting.

However, cutting slots in the hollow section and welding

stiffeners adds to the fabrication cost. Research by the

Steel Construction Institute for Corus (30) showed an

un-stiffened connection designed to BS 5400: Part 3 to

have a higher buckling resistance than that calculated

even when a lower flexibility value is used.

The failure loads calculated were relatively insensitive to

the actual value of connection stiffness. This showed

the use of diagonal stiffeners does not significantly add

to the global strength of tubular U-frame footbridges.

Where a steel floor plate is used it normally acts as the

“bracing” to the bottom chords, to carry the lateral

shear (mainly wind forces) back to the supports. If a

non-participating form of floor is used, cross bracing in

the plane of the bottom chord, to resist lateral forces,

must be considered.

Through trusses, used in longer spans, give lateral

stability to the top compression chord by means

of bracing in the plane of the top chord. Such bracing

will also share in the carrying of any lateral forces,

especially where the truss is clad on its sides and thus

subject to significant wind loads. At the ends of the span

these lateral forces have to be carried down to bearing

level through portal action or through a braced frame.

Members and connections – Vierendeel girders

In footbridges, Vierendeel girders normally use

rectangular hollow sections for greater stiffness

and strength at the connections between verticals

and chords.

The nature of vierendeel action is that vertical shear is

carried by shear/bending action of each length of chord,

and the vertical members are subject to complementary

horizontal shear and bending. Since shear is highest at

the ends of the span, the “fixed end moments” are

highest there also. The vertical members therefore need

to be strongest at the ends of the span.

On the other hand the central portions of the chords

sustain predominantly axial load, whilst the ends sustain

predominantly bending load. There is less need to vary

the size of the chord members, and usually only

thickness is varied, if at all.

The consequences are that the vertical members are

often wider (in the plane of the girder) at the ends of the

span and are sometimes closer together, variations

which are clearly visible in silhouette.

The strength of the joint between chord and vertical

members must be adequate to transmit the fixed end

moments. To do this both should have the same width

(normal to the plane of the girder). Under the higher

moments on the joints toward the ends of the span a

simple square joint may have inadequate strength, and

either triangular fillets (cut from the same section as the

vertical) or reinforcing plates may need to be added to

increase stiffness and strength (see Figure 6). The

appearance of these additions may not always be

acceptable and heavier sections may be preferred.

Stability of the compression chord again requires

U-frame action of the cross section and this again

requires adequate stiffness and strength of the

cross-member to vertical connection at the bottom

chord. Even with the heavier sections usually required

for a vierendeel girder, it may be necessary to insert

diagonal plates, as mentioned previously.

Figure 6: Detail of a haunched joint in a vierendeel girder

10 thickinsert plateslotted intochord

100 x 100 10 RHS

Weldgroundflush

Figure 5: Detail of diagonal plate through bottom chord



Right:Stiffened plate floor constructionFar right:Typical floor construction

Floor construction

The floor of a truss or vierendeel girder footbridge will

usually be of steel plate, though precast planks have

been used with trusses. The lighter steel deck is now

generally preferred.

The plate, typically 6mm or 8mm thick, is supported on

and welded to steel cross-members between the

chords. These cross-members form part of the U-frames

which stabilise the top chord and are themselves usually

hollow sections. The plate panels between chords

and cross-members are divided transversely and

sometimes longitudinally by stiffeners (usually flats) to

give added support.

On top of this plate a waterproof layer is required for

corrosion protection, and to give a non-slip surface for

safety. This is usually achieved with a thin membrane

(which acts both as waterproofing and as a binder) and a

surface dressing of fine aggregate. The total thickness is

about 4mm. This surface is often applied in the works

and does not add significantly to erection weights.

When precast planks are used it is necessary to provide

a shelf angle on the inner face of the chords on which

the planks can sit. It is very important that the joint

between concrete and steel is properly sealed or it could

become a moisture and corrosion trap.

Where drainage over the edges of the bridge is not

permitted, arrangements must be made to carry

rainwater to the ends of the bridge and then to drains or

a soakaway. A vertical curve or longitudinal camber

should be provided on a bridge which otherwise would

be level.

Where rainwater can be allowed to run off the side of the

bridge (for example over a river), the floor may be slightly

cambered transversely to facilitate drainage. With

stiffened thin steel plate decks, care also needs to be

exercised that panels do not dish between stiffeners and

allow ponding of water – the spacing of stiffeners is

usually limited for this reason. Weld sizes should be kept

to a minimum, to reduce distortion from welding.

(see GN 2.10 (31))





Parapets

Parapets are normally designed to comply with a

DMRB standard (see section 4.2). The parapet may be

either a separate item or may be combined with

structural members.

For trusses, the parapet is provided as separate units

fixed to the inside faces of the truss diagonals. The

diagonals must then be designed to carry lateral loads

from the parapet, and the parapet rails must be

designed to span between the diagonals which support

them. Parapet posts can alternatively be fixed to the

footway deck, though the attachment would need to be

strong enough to withstand the overturning moment

arising from lateral forces on the top rail.

Where vierendeel girders are used it is convenient to fix

parapet panels in the rectangular panels of the girders,

effectively using the vertical members as parapet posts.

This achieves an integrated appearance and produces a

slightly lesser overall width of bridge than with separate

parapets on the inner faces of the girder. The top chord

of the girder may also function as the top parapet rail, or,

if it is higher than the required parapet height, a separate

rail can be provided in addition to the top chord.

Cladding

Over rail tracks, the highway and rail authorities require

that solid non-climbable cladding be provided on the

inside face of the truss or vierendeel girder. This is

usually achieved by profiled steel sheeting, rigidised

aluminium, GRP panels or even flat sheets. Fine mesh

(maximum 50mm apertures) may be used over non-

electrified lines. Although the cladding is only required

over the tracks, a better appearance is often achieved

by providing the cladding over the full length of the

span. Great care needs to be exercised in detailing the

cladding, to avoid the creation of small inaccessible

sheltered ledges on the top of the lower chord where

moss and debris can accumulate or which may be used

for handholds or footholds.

Left:Parapets in vierendeel girder, HoramRight:In-line splice detailFar right:Erection of Christchurch footpath



Supports

Trusses and vierendeel girders are supported either on

bearings (if they span between concrete abutments, for

example) or directly on top of a simple steel

substructure without any bearings.

At abutments the point of support is normally directly

below the end vertical or diagonal members and thus

does not give rise to local bending of the chord section.

Other supports should also preferably be arranged

similarly. Where it is not convenient to do so, for

instance when a top landing cantilevers a short distance

beyond the support columns and the support is midway

between bracing connections, the bottom chord is

subjected to bending. It is then common to use a

heavier chord section over the last one or two panels of

the truss (see photograph below right).

Fabrication of trusses

Fabricators who specialise in hollow section fabrication

are familiar with all the types of detail needed for truss

footbridges and have appropriate equipment, such as

profile cutting equipment for tubulars etc.

A wide range of sizes of hollow sections is available

from the rolling mills, but it must be remembered that

the fabricator has to purchase material for each job,

either from the mill or from a stockist, and his orders

may be subject to minimum quantities and premiums for

small quantities. The designer should therefore try as far

as possible to standardise his choice of section size and

material grade.

Erection

Fortunately, most footbridges can be fabricated as a

complete length of the span and then transported, with

spans up to about 45m. Although fabrications over 27m

in length require special permission to travel on the public

highway, most fabricators prefer to complete fabrication

in the works wherever possible and are familiar with

arrangements for the movement of long lengths.

Bolted hollow section flanged joint details can be used

for site splices, though it may be felt that flange plate

end connections are somewhat cumbersome in

appearance. In-line splice details are much less

obtrusive, but require more effort in design and

fabrication (see photograph below left). In most cases,

spans must be complete before lifting, because closure

or possession periods will be very short.



3.4 Steel beam bridgesTypes of construction

Four types of construction are considered in this

section:

• a pair of steel beams with a non-structural floor on top

(e.g. timber)

• a pair of steel beams with a structurally participating

steel floor plate

• a steel box girder

• a half-through plate girder bridge as developed by

British Rail

The first three are appropriate where depth of

construction is not important. The fourth is appropriate

where minimum construction depth is critical.


For the relatively light loading on a footbridge, the depth

of beam in all cases can be arranged to be about 1/30

of the span. A typical bridge over a river or canal might

then have a span of 30m and a beam depth of 1m.

A simple I-beam bridge with non-structural floor might

comprise two girders about 1.5m apart on which is fixed

a floor of, in some instances, timber planks. Parapet

posts would be fixed to the top flange or the outer face

of the steel beams.

Steel girders with a structural participating steel floor

plate would be of similar overall proportions. Parapets

would be fixed on top of the floor plate.

With both forms, the girders can have a clean web over

their full length, as web stiffeners are needed only at

supports and on the inner faces for attachment of

bracing. The structural element therefore looks clean

and simple. The appearance will be influenced strongly

by the treatment of the parapet rails, posts and any

other feature added to the bridge. The use of simple

parapet details will contribute to a good non-fussy

overall appearance.

In some circumstances a distinct curvature in elevation

(more than would suffice just to aid drainage to the

ends) will add character to the appearance.

The use of a steel box girder extends the clean lines to

the soffit of the bridge. It can be complemented by a

simple basic parapet or can be contrasted by

embellishment with ornate fixtures and fittings. Typically

the box would be about 1.0m wide, with short steel

cantilevers either side to provide the necessary width.

Half-through plate girder bridges will usually have their

U-frame stiffeners on the outside faces and generally

look more heavy. Nevertheless, the half-through plate

girder bridge developed by British Rail (see page 22)

achieves a pleasing appearance.

Members and connections – I-beams/girders

For economical design, the pair of beams need to be

braced together to stabilise them against lateral

torsional buckling. Bracing at several positions in the

span will be necessary, roughly at 15 to 20 times the top

flange width to achieve reasonable limiting stress levels.

Bracing can simply be an X brace with single tie at each

position, bolted to stiffeners on the inside faces of the

webs. For the main girders, fabricated I-sections are

likely to be lighter and more economic than Universal

Beams. Castellated beams can provide a weight saving

in some circumstances whilst offering an interesting and

different appearance.

Left:Footbridge using rolled sections, SwaleRight:Footbridge with timber deck and parapetsFar right:Box girder footbridge and cycleway, Gablecross



A non-structural deck, such as timber planking, can be

simply bolted down to the top flange of the I-beams.

Particular attention should be paid to detailing, to

minimise crevices where dirt and moisture can

accumulate.

In many instances steel plate is used for the floor of the

bridge. The plate, typically about 6mm or 8mm thick, is

usually welded to the main girders and can therefore be

assumed to act structurally with them. Cross-members

will be required to carry the floor loading to the main

beams and these are sometimes extended by short steel

cantilevers outside the beam web, in which case an

edge beam is provided to give a neat face and to give

support to the parapet. A thin waterproof wearing

surface is normally specified, dressed with fine

aggregate for grip and durability. The surface is often

applied in the works.

Members and connections – box girders

Box girders are essentially similar to the paired plate

girders with steel deck, as described above, except that

the bottom flange joins the two webs and encloses the

space between. They are usually considered only for

spans over about 30m. The thickness of the top flange

which also forms the floor plate will be determined by

overall bending strength rather than local floor loading.

The plate is typically supported by transverse stiffeners

which cantilever to edge beams. Two or three

longitudinal stiffeners may be provided to stiffen the floor

plate when acting as the compression flange of the box.

Diaphragms are needed at supports and are often

provided at several positions along the length of the

girder (typically the third points) to control distortion.

Large holes will be required in the diaphragms if access

is required during fabrication or maintenance.

To improve appearance it is common to use slightly

sloping webs, creating a trapezoidal cross section.

The use of steel box girders has the advantage of

torsional strength and stiffness. They can be used in

continuous construction to simplify supports or to curve

the bridge in plan when desired for appearance. In a

straight bridge, torsional restraint (usually by means of

twin bearings) is needed only at the ends: a single

bearing will suffice at intermediate supports, thus

allowing the use of a single slender column.

Figure 7: Cross section through a typical box girder footbridge



Members and connections – half through girders

Half through plate girder footbridges are often used over

railways. The solid web provides the required screening

without the need for any non-structural additions. This

form has developed from the half-through plate girder

concept often seen in railway bridges. A particular form

developed by the former Midland Region of British Rail

is illustrated in photographs shown above. Two features

to note are: the use of a hollow section as top flange,

turned through 45° it forms a steeple cope, which

discourages walking along the flange; the absence of

any projection of the bottom flange prevents climbing

along the outer face.

U-frame action is provided by the flat intermediate

stiffeners to web and bottom flange. Typically they are

provided about every 1.5m.

Parapets

Where there are no cantilevers the parapet can either be

fixed to the top flange of the box or to the web of the

girder. The attachment positions should coincide with

bracing or cross-members, to provide restraint against

rotation under lateral loads on the parapet rail.

Where there are cantilevers, either the posts should

coincide with the cantilever positions or they should be

mounted on a torsionally stiff hollow section edge beam.

Fabrication

Whether using rolled I-beams or fabricated I-section

girders, the processes of drilling holes, adding stiffeners

etc. poses no difficulty to the fabricator. The fabricated

I-section can either be made using jigs and semi-

automatic welding or by a T and I automatic welding

machine. Curvature in elevation is easily achieved with

fabricated girders, and universal beams can readily be

curved by specialist bending companies prior to

fabrication. Fabrication of box sections requires more

traditional methods, and the completion of the closed

box makes it almost essential for manual work internally.

Details should be arranged for ease of access for work

and inspection.

Splices

For spans up to around 40m, it is quite likely that the

beams would be transported full length and splices

would not be needed. Over 40m they would be split

into at least two lengths; site connections would

normally be bolted.

Bolted splices are quite conventional, with few problems.

If a completely clean face is sought,it will be necessary

to have a site welded joint.


3.5 Composite beam bridgesTypes of construction

Composite construction is seen in footbridges in two

forms – a concrete slab on top of two I-girders or a

concrete slab on top of a closed steel box girder. The

open steel box form with slab which is sometimes used

in highway bridges is not normally seen in footbridges

Slabs may be cast insitu, though the relatively modest

extent of the shear connection and lighter design loads

on the slab allow greater opportunity to employ pre-cast

slabs. Such slabs are provided with open pockets to fit

over the shear connectors. The pockets and the joints

between slab sections are filled with concrete to create

the necessary structural continuity.


Composite footbridges typically have a span/depth ratio

of about 20 (depth measured from top of slab to

underside of girder).

Short cantilevers outside the lines of the webs will give

a better appearance, in the same way as they do for

highway bridges. A small upstand is needed at the

edges to provide a mounting for the parapets and to act

as a drainage upstand. A thick edge beam would create

a rather heavy appearance.

Members and connections

Composite construction produces a much heavier

structure than an all-steel footbridge; the dead

load accounts for over half of the total load in most

cases. The extra weight and consequent stiffness of this

form of construction has the advantage of being less

responsive to dynamic excitation.

Where transverse joints between precast units are not

designed to carry transverse shear, plan bracing will

also be needed.

Floor construction

Reinforced concrete slabs for footbridges are typically

about 150mm thick. They can be constructed insitu on

falsework or by using precast slabs.

Sometimes they can be cast in the fabrication yard, and

the complete composite structure transported to site

and erected.

A waterproofing membrane is required, plus some form

of durable wearing surface. A combined membrane and

wearing course with aggregate dressing, similar to that

used on steel decks, can be used.

Parapets

As for other forms of construction, parapets must

comply with DMRB or Network Rail requirements.

The parapet posts are fixed to the concrete slab or edge

beam with conventional holding down bolts.

Opposite page:Half through plate girder footbridge, Network RailAbove:Composite curved ‘I’ beam footbridge, Washington



3.6 Cable stayed bridgesFootbridges carry only relatively light loading. However,

when the main span is long, the requirements of

supporting its own dead load and of providing a

sufficiently stiff structure lead toward a much more

substantial structure than would seem appropriate for a

“mere” footbridge. As a result, an increasingly popular

solution for longer spans is the use of a cable stayed

arrangement. This effectively divides the span into shorter

lengths, for which lighter beams can be used. The pylons

for these bridges also add a strong visual feature which is

often welcomed.

Types of construction

Cable stays can be used with any of the forms of

construction previously described, though to complement

the light appearance, a slim form of deck construction is

likely to be more appropriate for all except the largest

spans. Supports can be provided to the main beams at

about 10m to 15m spacing, which facilitates the use of a

slender deck.

For most footbridges, twin planes of cable stays will

normally be used, one to each side of the bridge deck. A

pylon at one end of the main span will suffice up to about

100m span. Very long spans may require the use of pylons

at both ends. 'A' frame pylons are popular, with the two

stay planes inclined. Alternatively, individual pylon legs for

each cable plane can be arranged, or a “goal-post”

arrangement can be used; the stays can then lie in a

vertical plane.

Usually, at least two forestays should be provided in each

plane – a single stay is hard to justify on economic or

appearance grounds. The minimum span for a cable

stayed bridge with two forestays is thus around 35m.

A single backstay is usually sufficient, anchored to the

girder at the abutment which supports the end of the

backspan. Further backstays are only needed if the

backspan is long and requires intermediate support. The

stays are normally anchored at floor level to longitudinal

beams. The beams need to be stiff and strong enough to

span between anchor points and they may need to be

fairly deep. A lighter appearance, with shallow beam/floor

depth, might be achieved by using a vierendeel girder and

half-through construction. Footbridge pylons are usually

steel box or circular sections, for slender appearance,

ease of construction and economy.

Members and connections

The cable stays will normally be made from wire rope or

spiral strand. Strands are made by winding together, or

laying up, a number of galvanised steel wires. Ropes are

made up of a number of small strands wound together.

Ropes and spiral strands have a lower effective modulus

than solid steel. Parallel wire strands are also available.

Advice should be sought from specialist manufacturers on

the selection of strands.



In the dead load condition the stays are effectively

prestressed. It is important to calculate accurately the

stretch of the stays in the dead load condition, so that

the correct geometry of the structure is achieved.

Provision should be made for length adjustment in the

stays, to accommodate tolerances and errors.

Stays must obviously be sufficiently strong to support

the beams, but often more significant for small bridges

is the need to provide sufficiently stiff supports to the

beams and to avoid slack stays which will be easily

vibrated.

With twin planes of stays, the natural arrangement for

the deck structure is with main beams at either edge, to

which the stays are attached. The floor then spans

transversely between the beams. A single plane of stays

can only be used where a torsionally stiff box girder is

provided; the stays would be attached on the centreline

of the bridge. This is not normally convenient for a

single footway.

As well as provision for adjustment in length during

installation, attachment details should also be arranged

such that any stay can be replaced if need be. It is good

practice to make sure that the anchorages are as strong

at ULS as the breaking load of the stays.

Under the action of live load the stays provide stiff

support to the main beams and they thus behave

essentially as continuous beams. Axial load is also

transmitted to the beams by the stays, so the beams

must be designed for the combined load effects.

For very long spans, the deflection under load changes

the geometry of the structure. If the sag of the stays is

significant they will act as non-linear springs. Both these

effects should be taken into account in the analysis.

Computer programs are available which automatically

take account of the non-linear effects of varying

geometry under load.

Whilst ropes and strand can last the life of the bridge,

experience has shown that they should be

inspected from time to time to check for corrosion and

fatigue, particularly at the lower ends. The stay

anchorages should be accessible for such inspection

and maintenance. The design should also be such that

any one stay can be removed and replaced.

Dynamic response

Cable stayed bridges are relatively flexible and are more

prone to oscillation under wind or under deliberate

excitation by users. An all-steel construction results in a

very low level of structural damping, which can allow the

oscillations to grow significantly. The dynamic response

of the bridge should therefore be checked carefully.

Artificial damping, such as tuned mass dampers, can be

provided if necessary.

Floor construction

Deck construction is usually of stiffened steel plate,

though timber or reinforced concrete are sometimes

used instead.

Far left:Cable stayed ‘I’ beam footbridge, CumbernauldLeft:Royal Victoria Dock Bridge, LondonRight:Cable stay anchorage

3.7 Access ramps and stairsWhere approach ramps or stairs are needed they are

usually structurally independent, except for the need to

be supported at the top end either on the footbridge

superstructure or on a common substructure support.

They can therefore be of a structurally different form.

However, it is generally preferable to achieve harmony

of appearance between the two and to use a similar

construction form.

Stairs usually require, at most, one intermediate support

beneath the landing at mid-flight. Ramps require more

supports and indeed are small bridges themselves. Even

for ramps, the number of intermediate supports should

be kept as small as possible, with spans of at least 10m.

Supports should also be as simple as possible – a

T-shaped column and crosshead should be sufficient

in most cases (provided that resistance to impact is

not necessary).

Where supports may be subject to impact loads, they

will need to be significantly more substantial. The

foundations will also have to be larger. In these

circumstances the designer can choose either

reinforced concrete columns or a robust steel structure.

Since landings are nominally level, care needs to be

exercised to avoid ponding of water and accumulation

of debris. Extra drain holes in these areas together with

a small fall will suffice.

Handrails must be provided on the inside faces of

parapets on stairs and ramps, for safety reasons. A

clear gap of at least 40mm is desirable between the rails

and any adjacent members.

Stairs normally have semi-open risers. Fully open risers

are not permitted by BD 29/03.

At the bottom of flights of stairs, details should be

chosen which avoid acute corners, since they can trap

debris. To avoid this, stairs can be supported just above

the bottom of the flight, so that there is a clear gap

between the underside of the stringers and ground level.



Below:Stairs showing open treads and handrailsRight:Scissor ramp

3.8 Bearings and expansion jointsThe provisions for restraint or the accommodation of

movement due to expansion or other reasons depends

very much on the general arrangement of the bridge,

ramps and stairs.

When the bridge spans between bankseats or

abutments, expansion joints are needed, and the

structure will sit on bearings. At one end the bearings

may be fixed longitudinally, but if laminated bearings are

used, both ends can be 'free', as long as the bearings

can transmit any longitudinal forces.

Expansion joints need to accommodate movement

ranges of about 20mm, depending on span. Even at

ends which are longitudinally restrained there has to be

some provision for movement at deck level, owing to

rotational movements under live load.

For footbridge expansion joints, a simple detail should

be chosen, one which does not collect dirt or debris and

which can be dismantled for maintenance if required. A

simple leaf plate fixed to the bridge on one side and

sliding on a second plate on the fixed side can usually

be arranged in most circumstances. Particular attention

should always be given to the avoidance of steps facing

uphill, even as little as 5mm, since they always tend to

accumulate material washed down by run-off.

Where the bridge spans between steel column supports,

no bearings are needed. The bridge is simply bolted

down to the tops of the columns. Expansion is

accommodated by flexing of the columns and no

expansion joints are needed.

Consideration should be given to fixing long ramps at

the bottom end. Maximum longitudinal movement at the

far end therefore occurs where the columns are tallest

and most able to accommodate it.

Stairs should preferably be fixed at the bottom and

bolted to column supports. This effectively provides a

restraint for any ramp or bridge connected to the top of

a straight flight.

For light all-steel bridges, all support details, bearings or

direct connections to columns, should be designed to

resist at least a nominal uplift.



Below:Expansion joint leaf plateRight:End bearing box girder


Design codes, standards and guidance

4. Design codes, standards and guidance

4.1 British StandardsIn most circumstances, the British Standard BS 5400 (1)

will apply to the design and construction of footbridges.

In some cases, possibly where the bridge is connected

to a building, BS 5950 (2) might be called for.

For design of steel and composite structures, the

following Parts of BS 5400 are applicable

Part 2 Specification for loads

Part 3 Code of practice for design of steel bridges

Part 4 Code of practice for design of concrete bridges

Part 5 Code of practice for design of composite bridges

Part 6 Specification for materials and workmanship, steel

These codes cover all aspects of design for footbridges

of beam and truss construction. Design of tubular joints

is not covered in detail within Part 3 – see section 4.4

for further guidance. Similarly, the design of cable stays,

the strands and their anchorages, are not covered by

these codes – refer to section 4.5 for guidance.

Dimensional and safety requirements for stairs are given

in BS 5395 (3). These requirements are amended slightly by

the departmental standard for footbridges.

4.2 Departmental standardsThe requirements of the four UK highways authority (the

Highways Agency, the Scottish Executive, the Welsh

Assembly Government and the Department for Regional

Development Northern Ireland) are set out in the Design

Manual for Roads and Bridges (DMRB). This manual is a

collection of individual standards (BD documents) and

advice notes (BA documents).

Each of the design code parts of BS 5400 is

implemented by a BD standard (4), and some of

these standards vary certain aspects of the part that

they implement (notably BD 37 for Part 2 and BD 16 for

Part 5). For footbridges, a particular point to note is that

the requirements in relation to loads resulting from

collision of vehicles with the structure have been

significantly modified. The impact loads and the

circumstances in which they should be applied are

specified in BD 60 & BD 37 (the DMRB version of BS

5400 Part 2) and an amendment to it. The provisions

relate to the impact loads on supports located within

4.5m of the edge of the carriageway and to

superstructures which have less than 5.7m clearance

above the surface of the carriageway.

Other standards and advice notes also relate to the

design of footbridges. Design criteria for footbridges are

given in BD 29 (5). Highway cross sections and headroom

are given in TD 27 (6). Selected information from these

two documents is included in section 3. Standard TD 27

specifies a minimum clearance for footbridges of 5.7m.

This avoids the necessity of applying the impact

requirements of BD 37 on the superstructure, which

would be particularly onerous on a light structure such

as a footbridge.

Where supports need to be close to the edge of the

carriageway, they are required to be provided with

protective plinths and designed for impact loads. Where

they can be kept back from the carriageway, perhaps to

span a footway beside the road, the consequent savings

in the cost of the substructure should be considered.

Supports between carriageways should also be avoided

(unless they can be located more than 4.5m from the

road, which is not usually feasible).

The design of parapets on footbridges is referred by

BD 29 to the Interim Rules for Road Restraint Systems

IRRRS). The IRRRS (7) is a Highways Agency document,

not currently part of the DMRB, although it does state

that it supersedes a number of DMRB documents, such

as the earlier BD 52/93. The IRRRS refers to BS 7818 (8),

which gives dimensional requirements, design

requirements and a specification for construction of

metal parapets, and it specifies the design loading

classes for rails, posts and infill.

4.3 Railway standardsNetwork Rail are particularly concerned with prevention

of unauthorised access and are legally obliged to fence

its boundaries. Network Rail and the Railway Safety and

Standards Board also have more stringent requirements

in relation to collision loads. Reference should be made

to GC/RC5510: Recommendations for the Design of

Bridges (27). The following comments are based on advice

given in recent projects.


Design codes, standards and guidance

In considering the prevention of unauthorised access,

not only must the pedestrian face of the bridge be

designed to be non-climbable, it must also be

impossible to climb along the outer face from the ends

of the bridge – this usually means that trusses are clad

either side of the diagonals at the ends. The top flanges,

chords or parapets must be arranged so that they are

impossible to walk along.

The zone within 4.5m of the outermost running rail is

considered a danger zone; if any support is located

within that zone, collision effects must be considered.

Any substructure column must be able to withstand an

impact load, and the superstructure must be able to

continue to carry some live load without support from

the column. Design recommendations are given in

GC/RC5510.

4.4 Design of hollow section jointsThe design of hollow section joints is not fully covered

by the requirements of BS 5400: Part 3. There is

however extensive background research into the

behaviour of tubular joints and various documents have

been published which provide guidance.

For triangulated structures, where the joints transmit

essentially axial loads from one member to another, the

design of the joint involves checks on (a) the adequacy

of the welds at the end of the member and (b) the

bending of the walls of the hollow sections (which are

subjected to out of plane forces).

Guidance literature is available both for circular sections

and for rectangular sections. General guidance is given

in CIDECT publications (9), (10) & (11) and guidance in relation

to BS 5950: Part 1 is given in a Corus publication. (12)

Design rules in both of these documents may be applied

using partial factors appropriate to BS 5400. Similar

rules will be included in EN 1993-1-8 (13).

The extent of guidance on the design of joints for the

moments associated with vierendeel action (or with

U-frame action) is more limited, though there has also

been research on this topic. A stiffer and more efficient

joint is achieved when the bracing member is the same

width (normal to the moment plane) as the chord

member. Design guidance for this type of joint can also

be found in a Corus publication (12). Adequacy of both

the bracing member and the chord member must be

checked. If necessary, reinforcement of the joint can

be designed.

4.5 Design of cable stayed andsuspension bridgesFor general guidance on the design of cable stayed

bridges, reference should be made to standard texts,

such as Walther (14) or Troitsky (15). These are

comprehensive books, but they do include specific

comment on footbridges with illustrated examples.

The provisions of BS 5400 do not cover in detail the

design of wire ropes or similar elements, nor is there any

other appropriate national code. The designer therefore

needs to base his detailed design on an empirical

approach, based on load effects calculated in the usual

manner according to BS 5400 and adopting the general

objectives of the code.

Details of the specification of wire ropes and strands

can be found by reference to BS 302 (16), and of the

sockets by reference to BS 463 (17). The cold drawn wire

used for ropes and strands does not have a linear

stress/strain relationship, with a definite yield plateau,

as does structural steel. The relationship is generally

smooth, with decreasing tangent modulus as load

increases. Design of stays has therefore been based

traditionally on permissible stresses calculated by

dividing the ultimate or breaking strength by a suitably

large factor (i.e. a working stress philosophy). In the

absence of formal codes on a limit state basis, division

of this strength by a partial factor γm of about 2.0 at

ULS, in conjunction with normal values of γƒ1

and γƒ3

gives results consistent with the traditional approach.

Guidance on the design of suspension bridges can be

found in texts such as Pugsley (18). The tensile elements

may be wire rope or strand, as for cable stayed bridges,

though high tensile steel rods may be used for the main

tension members.


Design codes, Standards and Guidance

4.6 Design of steel and compositebridge beamsGuidance on the design of composite highway bridges

is given in a series of publications by The Steel

Construction Institute (19). These can be used as general

guidance in the design of footbridges in accordance

with BS 5400, both for composite beam and all-steel

beam designs.

Guidance on a wide range of practical aspects related to

steel bridge construction is given in a series of Guidance

Notes produced by the Steel Bridge Group (31).

4.7 Dynamic responseLimitations on the dynamic response of footbridges are

given in HA standard BD 37. The vertical natural

frequency of many footbridges will be below 5Hz and

the response must be checked. If the horizontal natural

frequency is less than 1.5Hz, checks must be made for

possible lateral excitation.

The susceptibility of a footbridge to aerodynamic

excitation has to be checked in accordance with

BD 49 (20). Bridges under 30m span are unlikely to be

susceptible. Detailed rules are given in BD 49 for

bridges that are susceptible.

4.8 Protective treatmentFor bridges subject to highways authority requirements,

the protective treatment specifications should be

selected from those listed in the guidance notes to the

Specifications for Highway Works (SHW) (21), (22). When

using those notes, access conditions should normally

be taken as “difficult”, which will result in use of metal

spray for the first coat. Galvanising may be suitable for

small components, such as parapets.

For Network Rail owned bridges, the protective

treatment and walkway surfacing must comply with

Network Rail line standard RT/CE/S/039 (28). Advice is

given in RT/CE/C/002 (29).

For other bridges, the HA specifications, or alternatives,

may be used, with the clients agreement.

In some circumstances, Weather Resistant Steels might

be used, provided that environmental constraints can be

met. (23), (24)

4.9 Steel materialsSteel material for plates, rolled sections and structural

hollow sections is covered by British Standards

EN 10025, EN 10210 (25). Information about the products

available from Corus (26) can be obtained from the Corus

Construction Centre. Contact details are on the back of

this brochure.


Flow charts

(Figure 5.2) (Figure 5.3) (Figure 5.4) (Figure 5.5)

Trusses and vierendeel girders Steel beams Composite beams

Choose structural form

Determine geometricconstraints

Scheme-specific details

Cable stayed bridges Ramps and stairs

Figure 5.1: Flow diagram for the design of footbridges

5. Flow charts

DMRB Standards for footbridges

DMRB Standards for highway

cross section and headroom

Far left:Renaissance Bridge, BedfordLeft:Smithkline Beecham, Marlow


Flow charts

Check adequacy at ULS

Check as a ‘truss’

Global analysis

Global analysis

Determine effectivelengths


Determine effectivelengths Check U-Frame action

Check adequacy of lateral bracing

Tension members

Compressionmembers

Longitudinal effects Lateral effects

Tension members

Compressionmembers

Triangulatedtruss?

Strength adequate?

Strength adequate?

Strength adequate?

Strength adequate?

Strength adequate?

Slender orcompact?


Satisfactory

Check adequacy at SLS

Yes

* For in-plane buckling, use the length between intersections (a); for out of plane buckling use (a) if there are effective lateral restraints or use 12.5.1 otherwise.

12.2.310.6.210.6.3

12.2.3No

Yes

10.6.1

12.412.5

11.5.29.9

I=a*12.5.1

Yes 10.6.210.6.39.9

Yes

Yes

12.211.5.1

12.1 12.6

12.5

Yes

Yes No

12.3


Figure 5.2: Flow chart for trusses and vierendeel girders

Check combinedbending and axial

effects


Flow charts

Check ULS momentand shear capacities

Satisfactory

YesNo

Yes

9.14

9.9.8

9.9

9.69.79.8

9.4

9.109.11

9.169.17

Yes

No Yes

Figure 5.3: Flow chart for steel beams

Check adequacy at SLS

Check bearingstiffeners

Unsymmetriccompactsection?

Check diaphragms and crossframes

All strengthsadequate?


Determine limitingstresses for LTB

Determine effectivesection

Determine limitingstresses and check

capacities

Box girder?

Global analysis


Flow charts

Satisfactory Satisfactory

9.9.89.9.5.2

Yes

9.14

5/5.2.4.25/5.2.64/4.1.1.1

No

Yes

5/6.1.24/4.8.3

9.9

Figure 5.4: Flow chart for composite beams


Check slab adequacy at ULS

Check bearing stiffeners

Unsymmetriccompact I-beam?

Check slab adequacy at ULS

Check beam adequacy at ULS

Global analysis

Check beam adequacy at SLS

Yes

Figure 5.5: Flow chart for cable stayed bridges

Determine dead loadprestress in stays

Check adequacy of cable stays

Check local effects at cable

anchorages

Check adequacy of pylon

Check adequacy of members as

trusses or beams

Global analysisNon-linear analysis ifdeflections or DL sag of stays are significant


Include effects during replacement

of each stay


References

6 References1. British Standards Institution

BS 5400: Steel, concrete and composite bridges – Parts 1 to 10,BSI, London (various dates)

2. British Standards InstitutionBS 5950, Structural use of steelwork in building, BSI, London

3. British Standards InstitutionBS 5395, Stairs, ladders and walkways, BSI, London

4. Highways AgencyDesign manual for roads and bridges, Volume 1 Section 3:BD 13, Design of steel bridges: use of BS 5400 Part 3;BD 16, Design of composite bridges:use of BS 5400: Part 5;BD 37; Loads for highway bridges,BD 60; The design of highway bridges for vehicle collision loads,The Stationery Office

5. Highways AgencyDesign manual for roads and bridges, Volume 2, Section 2, BD 29Design criteria for footbridges, The Stationery Office

6. Highways AgencyDesign manual for roads and bridges, Volume 6 Section 1, TD 27Cross-sections and headroom, The Stationery Office

7. Highways AgencyInterim Requirements for Road Restraint Systems (IRRRS), TheHighways Agency, 2002 (contact the Highways Agency for copies)

8. British Standards InstitutionBS 7818:1995 Specification for pedestrian restraint systems inmetal

9. CIDECTDesign guide for circular hollow sections (RHS) underpredominantly static loading, Verlag TÜV, Cologne, 1991

10. CIDECTDesign guide for rectangular hollow sections (RHS) joints underpredominantly static loading, TÜV, Cologne, 1992

11. CIDECTStructural stability of hollow sections, Verlag TÜV, Cologne, 1992

12. Corus TubesDesign of SHS welded joints, CT16, Corus Tubes, Corby 2001

13. British Standards InstitutionprEN 1993-1-8, Design of Steel Structures, Design of Joints,December 2003

14. Walther, R. et al,Cable stayed bridges, Thomas Telford, London, 1988

15. Troitsky, M. S.,Cable-stayed bridges, BSP, Oxford, 1988

16. British Standards InstitutionBS 302, Stranded steel wire ropes, BSI, London

17. British Standards InstitutionBS 463: Part 2:1970 Specification for sockets for wire ropes (metric units), BSI, London

18. Pugsley, A.The theory of suspension bridges, Edward Arnold, London, 1957

19. Iles, D. C.Design guide for composite highway bridges (P289)Design guide for composite highway bridges: Worked examples (P290) The Steel Construction Institute, 2001

20. Highways AgencyDesign manual for roads and bridges, Volume 1, Section 3, BD 49,Design rules for aerodynamic effects on bridges, The StationeryOffice

21. Highways AgencyManual of contract documents for highway works, The StationeryOffice; Volume 1: Specifications for highway works series 1900,Protection of steel against corrosionVolume 2: Notes for guidance on the specification for highwayworks,Series NG1900, Protection of steelwork against corrosion

22. CorusCorrosion Protection of Steel Bridges, 2002

23. Highways AgencyDesign manual for roads and bridges, Volume 2, Section 3, BD 7,Weathering steel for highway structures, The Stationery Office

24. CorusWeathering Steel Bridges, 2002

25. British Standards InstitutionBS EN 10025: 2004, Hot rolled products of structural steels. BS EN 10210, Hot finished structural hollow sections of non-alloyand fine grain structural steels, Part 1: 1994 Technical deliveryrequirements.

26. CorusProduct & Technical brochuresStructural sectionsStructural platesStructural hollow sections

27. Railway Safety and Standards BoardGroup StandardGC/RC5510: Recommendations for the Design of Bridges

28. Network RailLine StandardRT/CE/S/039; Specification RT98 - Protective Treatment forRailtrack Infrastructure

29. Network RailLine StandardRT/CE/C/002: Application and Reapplication of protectivetreatment to Railtrack Infrastructure

30. Corus TubesConnection flexibility in tubular U frame footbridges RT 451, December 1994

31. Evans, J. E. and Iles, D. C.Steel Bridge Group: Guidance notes on best practice in steel bridgeconstruction (P185), The Steel Construction Institute, 2002

Care has been taken to ensure that thisinformation is accurate, but Corus Group plc,including its subsidiaries, does not acceptresponsibility or liability for errors orinformation which is found to be misleading.

Copyright 2005Corus

Designed and produced by Orchard Corporate Ltd.

www.corusgroup.com

Corus Construction & IndustrialPO Box 1 Brigg RoadScunthorpeNorth LincolnshireDN16 1BPT +44 (0) 1724 405060F +44 (0) 1724 404224E [email protected]

English language version CD:3000:UK:01/2005

Documents

21044699 Design of Steel Footbridges 2005